The applicant of the present disclosure previously has proposed, in patent document JP-A-2002-160581, an AFS to improve the safety of an automobile while traveling. As shown in the conceptual diagram of FIG. 1, an automobile (CAR) has a steering sensor 1A for detecting a steering angle of a steering apparatus and a vehicle speed sensor 1B for detecting a vehicle speed. The detection outputs of the sensors 1A, 1B are provided to an electronic control unit (“ECU”) 2. The ECU 2 controls the irradiation area, e.g., the irradiation direction of head lamps 3 (i.e., swivel type lamps 3R, 3L) provided at the right and left front portions of an automobile so as to deflect in the left and right directions based on the received detection outputs to change the light distribution characteristics. According to the AFS, when an automobile travels on a curved road, it is possible to deflect the irradiation direction of the swivel type lamps in accordance with the steering angle of the automobile to illuminate the portion of the road ahead of the curve, thereby improving travel safety.
In such an AFS, when the left and right swivel type lamps are controlled to deflect only in accordance with the steering angle, various problems may occur. Trials have been performed in an attempt to eliminate such problems. For example, patent document JP-A-8-301005 proposes a technique in which the changing rate of the deflection of the swivel type lamps is made sufficiently large to control the deflection angle to a large extent when traveling at a high speed, and a portion of the road ahead of the vehicle (in the direction of travel) is irradiated earlier as the speed becomes higher to improve visibility. That patent document also proposes a technique to eliminate an incongruity resulting from the delay of the deflection operation of the swivel type lamps relative to the steering operation when the vehicle moves from a straight road to a curved road. According to that patent document, the swivel type lamps are deflected at a small changing rate even in an area where a steering angle is small such as when traveling state on a straight road.
In the technique disclosed in patent document JP-A-8-301005, the deflection angle of the swivel type lamps is made large during high speed travel of the automobile. However, when traveling on a road, such as a superhighway, with a large radius of curvature, automobiles typically travel at a high speed compared to their speed when traveling on a road with a small radius of curvature. Thus, when the deflection angle of the swivel type lamp is set to a large value during travel at a high speed, the deflection angle may become too large compared to the radius of curvature of the road. Thus, the lamps irradiate an area ahead of the automobile, but off the road. In such cases, the road is not irradiated suitably. Further, according to the technique of patent document JP-A-8-301005, when traveling in a state that is close to the straightly traveling state, although the rate of change of the deflection angle of the swivel type lamp is made small with respect to the steering angle, the deflection operation is not completely stopped. Thus, as described in patent document JP-A-8-3010052, the swivel type lamps may be deflected frequently, which may bother the driver. In particular, when traveling on a superhighway, the vibration from the road surface is applied to the steering, and the steering may vibrate with a small steering angle. In accordance with such a vibration, the swivel type lamps are deflected by a small angle, and it is difficult to eliminate the uncomfortable feeling that may be caused to the driver.